Optical compensator and liquid crystal display II

ABSTRACT

The invention that relates to an optical compensator for liquid crystal displays comprising: at least one O plate retarder; at least one planar A plate reader; at least one negative C plate retarder; and further relates to a liquid crystal display comprising such a compensator.

FIELD OF THE INVENTION

The invention relates to an optical compensator for liquid crystaldisplays and to a liquid crystal display comprising such a compensator.

BACKGROUND AND PRIOR ART

Optical compensators are used to improve the optical properties ofliquid crystal displays (LCD), such as the contrast ratio and the greyscale representation at large viewing angles. For example inuncompensated displays of the TN or STN type at large viewing anglesoften a change of the grey levels and even grey scale inversion, as wellas a loss of contrast and undesired changes of the colour gamut areobserved.

An overview of the LCD technology and the principles and methods ofoptical compensation of LCDs is given in U.S. Pat. No. 5,619,352, theentire disclosure of which is incorporated into this application by wayof reference.

As described in U.S. Pat. No. 5,619,352, to improve the contrast of adisplay at wide viewing angles a negatively birefringent C-platecompensator can be used, however, such a compensator does not improvethe greyscale representation of the display. On the other hand, tosuppress or even eliminate grey scale inversion and improve the greyscale stability U.S. Pat. No. 5,619,352 suggests to use a birefringentO-plate compensator. An O-plate compensator as described in U.S. Pat.No. 5,619,352 includes an O-plate, and may additionally include one ormore A-plates and/or negative C-plates.

The terms ‘O-plate’, ‘A-plate’ and ‘C-plate’ as used in U.S. Pat. No.5,619,352 and throughout this invention have the following meanings. An‘O-plate’ is an optical retarder utilizing a layer of a positivelybirefringent (e.g. liquid crystal) material with its principal opticalaxis oriented at an oblique angle with respect to the plane of thelayer. An ‘A-plate’ is an, optical retarder utilizing a layer ofuniaxially birefringent material with its extraordinary axis orientedparallel to the plane of the layer, and its ordinary axis (also called‘a-axis’) oriented perpendicular to the plane of the layer, i.e.parallel to the direction of normally incident light. A ‘C-plate’ is anoptical retarder utilizing a layer of a uniaxially birefringent,material with its extraordinary axis (also called ‘c-axis’)perpendicular to the plane of the layer, i.e. parallel to the directionof normally incident light.

As an O-plate retarder for example an optical retardation film(hereinafter abbreviated as ORF) comprising a layer of a liquid crystalor mesogenic material with tilted or splayed structure can be used.

As an A-plate retarder for example a uniaxially stretched polymer film,like for example a stretched polyvinylalcohol (PVA) or polycarbonate(PC) film, can be used. Alternatively, an A-plate retarder may comprisefor example a layer of a positively birefringent liquid crystal ormesogenic material with planar orientation.

As a negatively birefringent C-plate retarder for example a uniaxiallycompressed polymer film can be used. Alternatively, a negativelybirefringent C-plate may comprise for example a layer of a liquidcrystal or mesogenic material with a planar orientation and a negativebirefringence. Typical examples of negatively birefringent liquidcrystal materials are various kinds of discotic liquid crystalcompounds.

In addition to U.S. Pat. No. 5,619,352, optical compensators comprisingone or more O plates are described in prior art in WO 97/44409, WO97/44702, WO 97/44703 and WO 98/12584, the entire disclosure of which isincorporated into this application by way of reference. WO 97/44703 andWO 98/12584 further suggest to use O plates in combination with a Aplate.

WO 97/44703 reports that the use of a compensator comprising a O platein combination with a A plate, wherein the principal optical axes ofboth ORFs are oriented at right angles to each other, allowsparticularly good compensation of a TN-LCD, as it simultaneously reducesthe angle dependence of the contrast and the grey scale inversion in thedisplay.

However, when using compensators as described in the above mentionedprior art in combination with liquid crystal displays, especially TN orSTN-displays, the improvements of the optical properties of the display,like contrast at wide viewing angles, grey scale level stability, andsuppression of grey scale inversion, are still far from sufficient formost applications.

Therefore, it is desirable to have available improved opticalcompensators to further improve the optical performance of LCDs.

Definition of Terms

In connection with optical polarization, compensation and retardationlayers, films or plates as described in the present application, thefollowing definitions of terms as used throughout this application aregiven.

For the sake of simplicity, the term ‘liquid crystal material’ is usedhereinafter for both liquid crystal materials and mesogenic materials,and the term ‘mesogen’ is used for the mesogenic groups of the material.

The terms ‘tilted structure’ or ‘tilted orientation’ means that theoptical axis of the film is tilted at an angle θ between 0 and 90degrees relative to the film plane.

The term ‘splayed structure’ or ‘splayed orientation’ means a tiltedorientation as defined above, wherein the tilt angle additionally vanesmonotonously in the range from 0 to 90°, preferably from a minimum to amaximum value, in a direction perpendicular to the film plane.

The term ‘low tilt structure’ or ‘low tilt orientation’ means that theoptical axis of the film is slightly tilted or splayed as describedabove, with the average tilt angle throughout the film being between 1and 10°.

The term ‘planar structure’ or ‘planar orientation’ means that theoptical axis of the film is substantially parallel to the film plane.This definition also includes films-wherein the optical axis is slightlytilted relative to the film plane, with an average tilt angle throughoutthe film of up to 1°, and which exhibit the same optical properties as afilm wherein the optical axis is exactly parallel, i.e. with zero tilt,to the film plane.

The average tilt angle θ_(ave) is defined as follows$\theta_{ave} = \frac{\sum\limits_{d^{\prime} = 0}^{d}{\theta^{\prime}\left( d^{\prime} \right)}}{d}$wherein θ′(d′) is the local tilt angle at the thickness d′ within thefilm, and d is the total thickness of the film.

The tilt angle of a splayed film hereinafter is given as the averagetilt angle θ_(ave), unless stated otherwise.

The term ‘helically twisted structure’ relates to a film comprising oneor more layers of liquid crystal material wherein the mesogens areoriented with their main molecular axis in a preferred direction withinmolecular sublayers, with this preferred orientation direction indifferent sublayers being twisted around a helix axis that issubstantially perpendicular to the film plane, i.e. substantiallyparallel to the film normal. This definition also includes orientationswhere the helix axis is tilted at an angle of up to 2° relative to thefilm normal.

The term ‘homeotropic structure’ or ‘homeotropic orientation’ means thatthe optical axis of the film is substantially perpendicular to the filmplane, i.e. substantially parallel to the film normal. This definitionalso includes films wherein the optical axis is slightly tilted at anangle of up to 2° relative to the film normal, and which exhibit thesame optical properties as a film wherein the optical axis is exactlyparallel, i.e. with no tilt to the film normal.

For sake of simplicity, an optical film with a tilted, splayed, lowtilted, planar, twisted and homeotropic orientation or structure ishereinafter being shortly referred to as ‘tilted film’, ‘splayed film’,‘low tilt film’, ‘planar film’, ‘twisted film’ and ‘homeotropic film’,respectively.

Throughout this invention, both a tilted and a splayed film will also bereferred to as ‘O plate’. A planar film will also be referred to as ‘Aplate’ or ‘planar A plate’. A low tilt film will also be referred to as‘low tilt A plate’. A twisted film will also be referred to as ‘twistedA plate’.

In tilted, planar and homeotropic optical films comprising uniaxiallypositive birefringent liquid crystal material with uniform orientation,the optical axis of the film as referred to throughout this invention isgiven by the orientation direction of the main molecular axes of themesogens of the liquid crystal material.

In a splayed film comprising uniaxially positive birefringent liquidcrystal material with uniform orientation, the optical axis of the filmas referred to throughout this invention is given by the projection ofthe orientation direction of the main molecular axes of the mesogensonto the surface of the film.

The term ‘film’ as used in this application includes self-supporting,i.e. free-standing, films that show more or less pronounced mechanicalstability and flexibility, as well as coatings or layers on a supportingsubstrate or between two substrates.

The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal ormesogenic compound’ should denote materials or compounds comprising oneor more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e.groups with the ability to induce liquid crystal phase behaviour. Thecompounds or materials comprising mesogenic groups do not necessarilyhave to exhibit a liquid crystal phase themselves. It is also possiblethat they show liquid crystal phase behaviour only in mixtures withother compounds, or when the mesogenic compounds or materials, or themixtures thereof, are polymerized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b depict uncompensated prior art TN-LCD devices.

FIGS. 2 a and 2 b depict compensated TN-LCD devices with compensatorsaccording to a preferred embodiment of the present invention.

FIG. 3 depicts a compensated TN-LCD device with a compensator accordingto a preferred embodiment of the present invention.

FIG. 4 a is an isocontrast plot of an uncompensated prior art TN-LCDdevice according to comparison example A.

FIGS. 4 b and 4 c are grey level diagrams of an uncompensated prior artTN-LCD device according to comparison example A in horizontal (4 b) andvertical (4 c) viewing planes.

FIG. 5 a is an isocontrast plot of an uncompensated prior art TN-LCDdevice according to comparison example B.

FIGS. 5 b and 5 c are grey level diagrams of a conventionaluncompensated TN-LCD device according to comparison example B inhorizontal (5 b) and vertical (5 c) viewing planes.

FIGS. 6 a, 7 a, 8 a, 9, 10, 11, 12, 13, 14, 15 and 16 a are isocontrastplots of an inventive compensated TN-LCD device according to example1-11, respectively.

FIGS. 6 b, 6 c, 7 b, 7 c, 8 b, 8 c, 16 b and 16 c are grey leveldiagrams of an inventive compensated TN-LCD device according to examples1-11, respectively, in horizontal (b) and vertical (c) viewing planes.

SUMMARY OF THE INVENTION

One aim of the present invention is to provide an optical compensatorwhich has improved performance for compensation of LCDs, is easy tomanufacture, in particularly for mass production, and does not have thedrawbacks of prior art compensators as described above. Other aims ofthe present invention are immediately evident to the person skilled inthe art from the following detailed description.

The inventors of the present invention have found that the abovedrawbacks can be overcome, and an optical compensator with superiorperformance for compensation of the optical properties of liquid crystaldisplays can be obtained by using a combination of at least one O plateretarder, at least one planar A plate retarder and at least one negativeC plate retarder.

When using an optical compensator according to the present invention inan LCD, the contrast at large viewing angles and the grey levelrepresentation of the display are considerably improved, and grey scaleinversion is suppressed. In case of coloured displays, the colourstability is considerably improved and changes of the colour gamut aresuppressed. Furthermore, a compensator according to the presentinvention is particularly suitable for mass production.

One object of the present invention is an optical compensator for liquidcrystal displays, characterized in that it comprises

-   -   at least one O plate retarder,    -   at least one planar A plate retarder,    -   at least one negative C plate retarder.

Another object of the invention is a liquid crystal display devicecomprising the following elements

-   -   a liquid crystal cell formed by two transparent substrates        having surfaces which oppose each other, an electrode layer        provided on the inside of at least one of said two transparent        substrates and optionally superposed with an alignment layer,        and a liquid crystal medium which is present between the two        transparent substrates,    -   a polarizer arranged outside said transparent substrates, or a        pair of polarizers sandwiching said substrates, and    -   at least one optical compensator according to the present        invention, being situated between the liquid crystal cell and at        least one of said polarizers,        it being possible for the above elements to be separated,        stacked, mounted on top of each other or connected by means of        adhesive layers in any combination of these means of assembly.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention relate to an opticalcompensator comprising at least one O plate and at least one planar Aplate as described above, wherein

-   -   the average tilt angle θ_(ave), in the O plate is from 2 to 88°,        preferably from 30 to 60°,    -   the tilt angle θ in the O plate varies monotonously in a        direction perpendicular to the plane of the film,    -   the tilt angle θ in the O plate varies from a minimum value        θ_(min) at one surface of the film to a maximum value θ_(max) at        the opposite surface of the film,    -   θ_(min) in the O plate is from 0 to 80°, preferably from 1 to        20°.

θ_(max) in the O plate is from 10 to 90°, preferably 40 to 90°,

-   -   the thickness d of the O plate is from 0.1 to 10 μm, in        particular from 0.2 to 5 μm, very preferably from 0.3 to 3 μm,    -   the thickness d′ of the planar A plate is from 0.1 to 10 μm, in        particular from 0.2 to 5 μm, very preferably from 0.3 to 3 μm,    -   the optical retardation dΔn of the O plate is from 6 to 300 nm,        in particular from 10 to 200 nm, very preferably from 20 to 120        nm,    -   the optical retardation d′Δn′ of the planar A plate is from 12        to 575 nm, in particular from 20 to 300 nm, very preferably from        30 to 200 nm,    -   the O plate and/or the planar A plate comprise a linear or        crosslinked liquid crystalline polymer,    -   the negative C plate comprises a chiral linear or crosslinked        liquid crystalline polymer,    -   the negative C plate is a bireferingent polymer film,    -   the negative C plate is a birefringent triacetylcellulose (TAC)        or diacetylcellulose (DAC) film.

Further preferred embodiments of the present invention relate to anoptical compensator comprising

-   -   one O plate, one planar A plate and one negative C plate,        especially preferably wherein the negative C plate is situated        between the O plate and the planar A plate,    -   one O plate, one planar A plate and two negative C plates,    -   one O plate and one planar A plate, at least one of which is        provided on a negatively birefringent substrate that serves as        negative C plate.

A further preferred embodiment of the present invention relates to aliquid crystal display comprising a liquid crystal cell, a pair ofpolarizers sandwiching the cell, and one inventive compensator asdescribed above and below located on each side of the liquid crystal(LC) cell between the cell and the polarizer.

Especially preferred are displays wherein

-   -   the LC cell is a twisted nematic or supertwisted nematic cell,    -   the optical axis of the O plate and the planar A plate are        oriented at right angles with each other,    -   the O plate is facing the polarizer and the planar A plate is        facing the LC cell,    -   in case the O plate is facing the polarizer, its optical axis is        parallel to the optical axis of the liquid crystal medium at the        nearest surface of the liquid crystal cell,    -   in case the O plate is facing the LC cell, its optical axis is        at right angles to the optical axis of the liquid crystal medium        at the nearest surface of the liquid crystal cell,    -   the O plate is situated with its low tilt surface facing the LC        cell,        Especially preferred compensator stacks for inventive displays        according to the preferred embodiments as described above are        shown in table 1. Therein, LC denotes a liquid crystal cell, O        denotes a tilted or splayed O plate, A denotes a planar A plate,        and −C denotes a negative C plate. For the case where the O        plate is a splayed O plate, the arrow is denoting the preferred        direction of increasing tilt angle.

For sake of simplicity, the polarizers are omitted in table 1. A displayfor practical applications will, however, further comprise a pair ofpolarizers sandwiching the stack as shown in table 1.

In the stack formats as shown in table 1 the single retarder componentsare arranged symmetrically, therefore incoming light may enter the stackfrom either side.

TABLE 1 Preferred compensator stacks in inventive displays [A] ←O −C ALC A C O→ [B] A −C O→ LC ←O −C A [C] −C A ←O −C LC −C O→ A −C [D] −C O→A −C LC −C A ←O −C [E] −C A −C O→ LC ←O −C A −C [F] ←O −C A −C LC −C A−C O→ [G] A −C ←O −C LC −C O→ −C A [H] −C O→ −C A LC A −C ←O −C [I] A −C−C O→ LC ←O −C −C A [J] ←O −C −C A LC A −C −C O→

Particularly preferred ar compensator stacks of type [H] as shown intable 1, wherein the absolute retardation values of the O plate and theA plate are approximately the same.

Further preferred are stacks of type [H] as shown in table 1, whereinthe optical axis of the O plate and the A plate are oriented at rightangles with each other, and oriented at an angle of from 1 to 15°,preferably from 5 to 10 a, relative to the polarization direction of thepolarizer and/or relative to the optical axis of the liquid crystalmedium at the nearest surface of the liquid crystal cell, respectively.

The inventive optical compensators can be used for compensation ofconventional displays, in particular those of the twisted nematic orsuper twisted nematic mode, such as TN, HTN, STN or AMD-TN displays, indisplays of the IPS (in plane switching) mode, which are also known as‘super TFT’ displays, in displays of the DAP (deformation of alignedphases) or VA (vertically aligned) mode, like e.g. ECB (electricallycontrolled birefringence), CSH (colour super homeotropic), VAN or VAC(vertically aligned nematic or cholesteric) displays, in displays of thebend mode or hybrid type displays, like e.g. OCB (optically compensatedbend cell or optically compensated birefringence), R-OCB (reflectiveOCB), HAN (hybrid aligned nematic) or π-cell displays.

Especially preferably the compensators are used for compensation of TN,HTN and STN displays.

In the following, the invention will exemplarily be described in detailfor compensation of a TN display.

FIG. 1 a depicts an uncompensated standard type TN display device in itsoff-state, i.e. when no voltage is applied, comprising a TN cell 1 witha liquid crystal layer in the twisted nematic state sandwiched betweentwo transparent electrodes (which are not shown here), and a pair oflinear polarizers 2,2′. The twisted nematic orientation of the liquidcrystal layer is schematically depicted by the mesogens 1 a. The dashedlines 1 b and 1 c represent the orientation direction of the mesogens 1a that are adjacent to the cell walls of the TN cell 1.

In the display device shown in FIG. 1 a, the polarization axes of thelinear polarizers 2,2′ are oriented at right angles to the optical axis1 b, 1 c of the liquid crystal medium at the nearest surface of theliquid crystal cell 1, respectively. This orientation of the polarizersrelative to the TN cell is hereinafter also generally referred to as ‘Emode’.

FIG. 1 b depicts an uncompensated standard type TN display device likethat of FIG. 1 a, but wherein the polarization axes of the linearpolarizers 2,2′ are oriented parallel to the optical axis of the liquidcrystal medium at the nearest surface of the liquid crystal cell 1,respectively. This orientation of the polarizers relative to the TN cellis hereinafter also generally referred to as ‘O mode’.

FIG. 2 a, b schematically depict compensated TN-LCD devices according topreferred embodiments of the present invention in the off-state, withFIG. 2 a showing a device in the O mode and FIG. 2 b showing a device inthe E mode, as explained above.

The devices consist of a TN cell 1 with a liquid crystal layer in atwisted nematic state sandwiched between two transparent electrodes(which are not shown here), a pair of linear polarizers 2,2′ and twocompensators, each compensator consisting of a splayed O plate 3,3′ aplanar A plate 4,4′, and two negative C plates 5,5′,5″ and 5′″ on eachside of the TN cell 1. The stack-formats of the optical components inFIG. 2 a, b correspond to type [H] of table 1 above.

In the devices examplarily shown in FIG. 2 a, b each of the O plates3,3′ and the planar A plates 4,4′ are provided directly on the negativeC plates 5,5′,5″,5′″ which serve as substrates for the O and A plates.

The stacks of optical components in the devices shown in FIG. 1 a, b andFIG. 2 a, b are symmetrical, hence incoming light may enter the devicefrom either side.

The O plates 3,3′ consist, as an example, of a layer of polymerizedliquid crystal material with splayed structure. The splayed structure isschematically depicted by the mesogens 3 a and 3 a′ which are orientedwith their main molecular axis tilted at an angle θ relative to theplane of the layer, wherein the tilt angle θ increases in a directionnormal to the film, starting with a minimum value θ_(min), on the sideof the O plate 3,3′ facing the TN cell 1.

The dashed lines 3 b and 3 b′ represent the projection of theorientation directions of the mesogens 3 a and 3 a′, respectively, indifferent regions of the O plates 3,3′ onto the surfaces of therespective O plates 3,3′. The dashed lines 3 b,3 b′ are also identicalwith the principal optical axis of the respective O plates 3,3′. In thedevices shown in FIG. 2 a, b, the principal optical axes of the O plates3,3′ are oriented parallel to the polarization direction of therespective adjacent linear polarizer 2,2′, and parallel to therespective adjacent orientation direction 1 b,1 c of the mesogens 1 a inthe TN cell 1.

The planar A plates 4,4′ consist, as an example, a layer of polymerizedliquid crystalline material with planar structure. The planar structureis represented by the mesogens 4 a, 4 a′ which are oriented with theirmain molecular axes parallel to the plane of the layer.

The dashed lines 4 b,4 b′ the orientation direction of the mesogens 4a,4 a′, which is identical with the principal optical axis of therespective A plate 4,4′. In the devices shown in FIG. 2 a, b, theprincipal optical axis 4 b,4 b′ of the planar A plate 4,4′ is orientedat right angles to the polarization direction of the respective adjacentlinear polarizer 2,2′, and at right angles to the respective adjacentorientation direction 1 b,1 c of the mesogens 1 a in the TN cell 1.

In the devices shown in FIG. 2 a, b, the mesogens at the surface of theO plate 3,3′ facing the TN cell 1 exhibit a planar orientation, i.e. theminimum tilt angle θ_(min) is substantially 0 degrees. However, othervalues of θ_(min) are also possible.

In the O plate according to the preferred embodiments as shown e.g. inFIG. 2 a, b, the minimum tilt angle θ_(min) is preferably from 0 to 80°,in particular from 1 to 20°, very preferably from 1 to 10° and mostpreferably from 1 to 5°. The maximum tilt angle θ_(max) in an O plateaccording to these preferred embodiments is preferably from 10 to 90°,in particular from 20 to 90°, very preferably from 30 to 90°, mostpreferably from 40 to 90°.

In the preferred devices shown e.g. in FIG. 2 a, b, the planar A plates4,4′ comprise a polymerized liquid crystalline material with a planarstructure, as represented by the mesogens 4 a,4 a′ which are orientedwith their main molecular axes parallel to the plane of the layer.

Apart from the preferred embodiments as depicted in FIG. 2 a, b, othercombinations and stack formats of the O plates and planar A plates arealso possible.

For example, in the preferred devices shown in FIG. 2 a, b, the O plate3 and the adjacent A plate 4, and/or the O plate 3′ and the adjacentplanar A plate 4′, are mutually exchangeable with each other.Furthermore, the compensators or entire ORF stacks on one side of the TNcell are mutually exchangeable with the compensators or entire filmstacks on the opposite side of the TN cell.

In the inventive devices exemplarily shown in FIG. 2 a, b, the opticalaxes 3 b,3 b′ of the O plates 3,3′ and the optical axes 4 b,4 b′ of thelow tilt A plates 4,4′ are either parallel or at right angles to theorientation direction 1 b,1 c of the mesogens 1 a in the TN cell 1 andto the polarization direction of the polarizers 2,2′.

In another preferred embodiment of the present invention, the opticalaxes 3 b,3 b′ of the O plates 3,3′ are twisted clockwise within the filmplane, at an angle +δ, and the optical axes 4 b,4 b′ of the low tilt Aplates 4,4′ are twisted counterclockwise within the film plane, at anangle −δ, relative to the optical axes of the the orientation direction1 b,1 c of the message ns 1 a in the TN cell 1 and to the polarizationdirection of the polarizers 2,2′. The absolute value of said twist angle±δ is preferably from to 1 to 15°, very, preferably from 5 to 10°.

Further to the preferred embodiments shown in FIG. 2 a, b, a compensatoraccording to the present invention may also comprise more than one Oplate and/or more than one A plate.

In case the inventive compensator comprises two or more O plates, theoptical axes of the O plates can be parallel one to another, or beoriented at an angle with one another. Preferably the optical axes ofthe O plates are oriented either parallel or at right angles to eachother.

In case an inventive compensator comprises two or more O plates, each Oplate can be arranged relative to the closest successive O plate suchthat their respective surfaces with minimum tilt angle θ_(min) arefacing each other, or such that their respective surfaces with maximumtilt angle θ_(max) are facing each other, or such that the surface of afirst O plate with minimum tilt angle θ_(min) is facing the surface ofthe closest successive O plate with maximum tilt angle θ_(max).

Further preferred arrangements of two or more O plates in an inventivecompensator are those as described in WO 98/12584, in particular thoseaccording to the preferred embodiments described in WO 98/112584 onpages 8-11 and in FIGS. 1 a, 1 b and 1 c.

In another preferred embodiment of the present invention, the opticalcompensator comprises one or more, especially preferably one or two,negative C plates. As a negative C plate, it is possible to use forexample a negatively birefringent plastic substrate on which the twistedand/or the O plate are provided.

The devices shown in FIG. 2 a, b comprise splayed O plates.Alternatively, it is possible to use tilted, but not splayed, O platesinstead of, or in addition to splayed O plates in the inventive LCdisplays. Preferably, however the inventive LC displays do comprise oneor more splayed O plates.

As an O plate for the inventive compensator it is possible to use anoptical film comprising a polymerized liquid crystal material withtilted or splayed structure, as described in the U.S. Pat. No.5,619,352, WO 97/44409, WO 97/44702, WO 97/44703 or WO 98/12584, withthe entire disclosure of these documents being incorporated into thisapplication by way of reference.

As an O plate, it is also possible to use a multilayer film comprisingtwo or more sublayers of polymerized liquid crystal material, with eachsublayer having a tilted structure with constant tilt angle, whereinsaid tilt angle increases or decreases monotonously from one sublayer tothe next sublayer throughout the multilayer.

In a preferred embodiment of the invention, the O plate is a tilted orsplayed optical retardation film (ORF) film as described in WO 98/12584,or a film prepared in analogy to the methods disclosed therein.According to the WO 98/12584, an ORF with tilted or splayed structurecan be obtained by coating a layer of a polymerizable mesogenic materialonto a substrate or between two substrates, aligning the material into atilted or splayed orientation, and polymerizing the material by exposureto heat or actinic radiation.

Alternatively it is possible to use as an O plate a liquid crystal filmas described in WO 96/10770, which is prepared from a polymerzableliquid crystal material with a smectic A or smectic C phase and anematic phase at higher temperatures. The polymerizable liquid crystalmaterial is applied in its nematic phase onto a substrate that is e.gcovered with an alignment layer of obliquely deposited SiO, and loweringthe temperature into smectic C phase of the material. This leads to anincreas of the tilt angle, as the material adopts its naturally tiltedsmectic C structure, which is then fixed by polymerization of the liquidcrystal material. The above preparation method and possible variationsthereof are described in detail in WO 96/10770, the entire disclosure ofwhich is incorporated into this application by way of reference.

It is also possible to use as an O plate an inorganic thin film with atilted microstructure, which can be obtained by oblique vapor depositionof an inorganic material, e.g. Ta₂O₅, as described in WO 96/10773.

As a planar A plate for the inventive compensator it is possible to usea uniaxially stretched polymer film of e.g. polyethyleneterephthalate(PET), polyvinylalcohol (PVA) or polycarbonate (PC). For example PETfilms are commercially available from ICI Corp. under the trade nameMelinex. Especially preferred are PVA and PET films.

Preferably the planar A plate is comprising a polymerized liquid crystalmaterial with a planar structure, as described in the WO 98/04651, theentire disclosure of which is incorporated into this application by wayof reference.

The thickness d of the O plate and the thickness d′ of the planar Aplate in each case independently is preferably from 0.1 to 10 μm, inparticular from 0.2 to 5 μm, most preferably from 0.3 to 3 μm. For someapplications, a film thickness between 2 and 15 μm is also suitable.

As negative C plate, it is possible to use a stretched or uniaxiallycompressed plastic film, as described e.g. in U.S. Pat. No. 4,701,028,or an inorganic thin film obtained by physical vapor deposition, asdescribed e.g. in U.S. Pat. No. 5,196,953.

Particularly preferred are inventive compensators wherein the O plate isprovided on a negatively birefringent substrate which serves as negativeC plate. Further preferred are inventive compensators wherein each ofthe twisted and the O plate are provided on a negatively birefringentsubstrate.

As a negatively birefringent film substrate for example a uniaxiallycompressed plastic film, like e.g. PET, PVA, PC, triacetylcellulose(TAC) or diacetylcellulose (DAC) can be used. Especially preferred arePVA, TAC and DAC films.

In a particularly preferred embodiment, the negative C plate is a filmcomprising one or more layers of anisotropic material having a highlytwisted structure, wherein the helical pitch has a value below thevisible wavelength range.

A highly twisted film, which has the structure of a twisted A plate asdefined above but with a high twist angle, exhibits a compensationperformance for liquid crystal displays that is at least equivalent to,and in some cases even better than, the performance of a conventionalnegatively birefringent C-plate retarder. In case the helical pitch ofthe highly twisted A plate is such that it shows selective reflectionlight of a wavelength below the visible range, the highly twisted Aplate can be used as a negative C plate in the inventive compensator.

This is an additional benefit of the present invention, since the stateof the art negatively birefringent C-plates in most cases either requirecomplicated manufacturing procedures such as vapour deposition of aninorganic thin film (as described for example in U.S. Pat. No.5,196,953), or they require the use of negatively birefringentmaterials, which are most often less easily available and more expensivethan positively birefringent materials.

Thus, FIG. 3 depicts a compensated TN-LCD device according to a furtherpreferred embodiment of the invention in its off-state, i.e. when novoltage is applied, in the E mode. The device contains a TN cell 1 witha liquid crystal layer in the twisted nematic state sandwiched betweentwo transparent electrodes (which are not shown here), and a pair oflinear polarizers 2,2′. The device further comprises on each side of theTN cell an inventive compensator consisting of an O plate 3,3′, a planarA plate 4,4′, a negative C plate 5,5′ and a highly twisted A plate 6,6′which serve as negative C plate, wherein the O plates 3,3′ are providedon the negative C plates 5,5′ which serve as substrates he stack formatof the optical components in FIG. 3 corresponds to type [H] of table 1above.

It should be noted that FIGS. 1-3 are not intended to depict the realproportions of the individual device components. Thus, for example in areal device according to FIG. 3 the negative C plates 5,5′ are expectedto have a higher thickness than the highly twisted A plates 6,6′, i.e.,different than suggested by the dimensions shown FIG. 3.

The highly twisted A plate preferably exhibits a chiral liquid crystalmaterial, e.g. a cholesteric material, with a highly twisted structurewherein the main molecular axes of the mesogens are helically twisted atmore than one full helix turn around an axis perpendicular to the planeof the film.

The twist angle φ of the twisted A plate can also be expressed by thehelical pitch p of the liquid crystalline material and the thickness d″of the A plate according to the equationφ=360°·d″/p

The helical pitch p of a highly twisted A plate in an inventivecompensator is preferably less than 250 nm, so that the film does notreflect visible light. Preferably the pitch p is from 50 to 250 nm, inparticular from 100 to 250 nm.

The thickness of a highly twisted A plate is preferably from 0.1 to 5μm, in particular from 0.2 to 3 μm, very preferably from 0.3 to 1.5 μm.

A highly twisted A plate according to this preferred embodimentpreferably comprises one or more layers of polymerized cholestericliquid crystal material as described for example in GS 2,315,072, inparticular as described therein on page 2-14 and in examples 1-5. Thesefilms do exhibit a very small helical pitch leading to a reflectionwavelength in the UV range. For the purposes of the present invention,highly twisted A plates with a pitch as described in the GB 2,315,072,most preferably with an even smaller pitch, are preferred. These filmscan be prepared according to or in analogy to the methods described inGB 2,315,072.

Alternatively, it is also possible to use as highly twisted A plate oneor more layers of platelets or platelet-shaped flakes comprising anoriented polymerized cholesteric liquid crystal material with planarorientation, these platelets or flakes being dispersed in alight-transmissive binder, and being oriented such that the helix axisof the cholesteric liquid crystal material extends substantiallyperpendicular to the plane of the layer. Suitable platelets or flakesare described e.g. in WO 97/30136 (Merck), WO 96/18129 (CRL), U.S. Pat.No. 5,364,557 (Faris), EP 0 601 483, EP 0 773 250 or U.S. Pat. No.5,827,449 (Wacker).

In a preferred embodiment of the invention, the highly twisted A plateis a film as described in GB 2,315,072, or a film prepared in analogy tothe methods disclosed therein, with the entire disclosure of thisdocument being incorporated into this application by way of reference.

Thus, according to GB 2,315,072 a highly twisted A plate can be obtainedby coating a layer of a chiral polymerizable mesogenic material onto asubstrate or between two substrates, aligning the material into atwisted orientation, wherein the helical twist axis is perpendicular tothe plane of the layer, and polymerizing the material by exposure toheat or actinic radiation.

In another preferred embodiment of the present invention the compensatoradditionally comprises one or more twisted A plates with low or moderatetwist, in particular with a twist angle below 360°. In these twisted Aplates, the twist angle φ is preferably from 90° to 270°. As a twisted Aplate, it is possible to use e.g. a twisted nematic polymerfilm-as-described in the EP 0 423 881 (Philips), EP 0 576 931 (Casio) orU.S. Pat. No. 5,243,451 (Ricoh).

In case of the twisted or highly twisted A plate, it is also possible touse a layer of a non-polymerized liquid crystal material. For example, anematic liquid crystal mixture can be used that is provided between twotransparent substrates and exhibits a planar twisted orientation,wherein the twist is induced by different orientation of the liquidcrystal molecules at the substrates, like in a standard type TN cell, orthe twist is brought about by one or more chiral dopants added to thenematic material. Alternatively a layer of a cholesteric liquid crystalmixture can be used.

As linear polarizer, a standard type commercially available polarizercan be used. In a preferred embodiment of the present invention thelinear polarizer is a low contrast polarizer. In another preferredembodiment of the present invention the linear polarizer is a dichroicpolarizer, like a dyed polarizer.

The individual optical components in the inventive compensators anddisplays, such as the liquid crystal cell, the individual retarders andthe linear polarizers, can be separated or can be laminated to othercomponents. They can be stacked, mounted on top of each other or beconnected e.g. by means of adhesive layers.

It is also possible that stacks of two or more retarders are prepared bycoating the liquid crystalline material of an retarder directly onto anadjacent retarder, the latter serving as substrate.

The optical compensator and/or the display device according to thepresent invention may further comprise one or more adhesive layersprovided to the individual optical components like the liquid crystalcell, the polarizers and the different retarders.

In case the polymerized liquid crystal material in the O plate and/orthe A plate is a polymer with high adhesion, separate adhesive layersmay also be omitted. Highly adhesive polymers are for example liquidcrystal polyepoxides. Furthermore, liquid crystal linear polymers orcrosslinked polymers with low degree of crosslinking show higheradhesion than highly crosslinked polymers. The above highly adhesiveliquid crystal polymers are therefore preferred for specificapplications, especially for those which do not tolerate additionaladhesive layers.

The inventive compensator may also comprise one or more protectivelayers provided on the surface of the individual optical componentsdescribed above.

In case of the twisted and highly twisted A plate, the polymerizablematerial comprises achiral polymerizable mesogenic compounds and furthercomprises at least one chiral compound. The chiral compounds can beselected from non-polymerizable chiral compounds, like e.g. chiraldopants as used in liquid crystal mixtures or devices, polymerizablechiral non-mesogenic or polymerizable chiral mesogenic compounds.

In case of the O plate and the planar A plate, the polymerizablematerial preferably consists essentially of achiral polymerizablemesogenic compounds.

Preferably a polymerizable mesogenic material is used that comprises atleast one polymerizable mesogen having one polymerizable functionalgroup and at least one polymerizable mesogen having two or morepolymerizable functional groups.

In another preferred embodiment the polymerizable material comprisespolymerizable mesogenic compounds having two or morel polymerizablefunctional groups (di- or multireactive or di- or multifunctionalcompounds). Upon polymerization of such a mixture a three-dimensionalpolymer network is formed. An optical retardation film made of such anetwork is self-supporting and shows a high mechanical and thermalstability and a low temperature dependence of its physical and opticalproperties.

By varying the concentration of the multifunctional mesogenic or nonmesogenic compounds the crosslink density of the polymer film andthereby its physical and chemical properties such as the glasstransition temperature, which is also important for the temperaturedependence of the optical properties of the optical retardation film,the thermal and mechanical stability or the solvent resistance can betuned easily.

The achiral and chiral polymerizable mesogenic mono-, di- ormultireactive compounds used for the instant invention can be preparedby methods which are known per se and which are described, for example,in standard works of organic chemistry such as, for example,Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart.Typical examples are described for example in WO 93/22397; EP 0 261 712;DE 19504224; DE 4408171 and DE 4405316. The compounds disclosed in thesedocuments, however, are to be regarded merely as examples that do notlimit the scope of this invention.

Examples representing especially useful monoreactive chiral and achiralpolymerizable mesogenic compounds are shown in the following list ofcompounds, which should, however, be taken only as illustrative and isin no way intended to restrict, but instead to explain the presentinvention:

Examples of useful direactve chiral and achiral polymerizable mesogeniccompounds are shown in the following list of compounds, which should,however, be taken only as illustrative and is in no way intended torestrict, but instead to explain the present invention

In the above formulae, P is a polymerizable group, preferably an acryl,methacryl, vinyl, vinyloxy, propenyl ether, epoxy or stytryl group, xand y are each independently 1 to 12. A is 1,4-phenylene that isoptionally mono- di or trisubstituted by L¹ or 1,4-cyclohexylene, v is 0or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or a single bond, Y is a polar group,R⁰ is an unpolar alkyl or alkoxy group, Ter is a terpenoid radical likee.g. menthyl, Chol is a cholesteryl group, and L¹ and L² are eachindependently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy,alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 Catoms.

The term ‘polar group’ in this connection means a group selected from F,Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinated carbonyl orcarboxyl group with up to 4 C atoms or a mono- oligo- or polyfluorinatedalkyl or alkoxy group with 1 to 4 C atoms.

The term ‘unpolar group’ means an alkyl group with 1 or more, preferably1 to 12 C atoms or an alkoxy group with 2 or more, preferably 2 to 12 Catoms.

In case of the preparation of the twisted A plate, the chiralpolymerizable mesogenic material may comprise one or morenon-polymerizable chiral dopants in addition or alternatively to chiralpolymerizable mesogenic compounds. Especially preferred are chiraldopants with a high helical twisting power (HTP), in particular thosedisclosed in WO 98/00428. Further typically used chiral dopants are e.g.the commercially available S 1011, R 811 or CB 15 (from Merck KGaA,Darmstadt, Germany).

Especially preferred are chiral non-polymerizable dopants selected fromthe following formulae

including the (R,S), (S,R), (R,R) and (S,S) enantiomers not shown,wherein E and F are each independently 1,4-phenylene ortrans-1,4-cyclohexylene, v is 0 or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or asingle bond, and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms.

The compounds of formula IIIa and their synthesis are described in WO98/00428. The compounds of formula IIIb and their synthesis are:described in GB 2,328,207.

The above chiral compounds of formula IIIa and IIIb exhibit a very highhelical twisting power (HTP), and are therefore particularly useful forthe preparation of a highly twisted ORF as used in the presentinvention.

The polymerizable mesogenic material is coated onto substrate, alignedinto a uniform orientation and polymerized according to a process asdescribed in WO 98/12584 or GB 2,315,072, thereby permanently fixing theorientation of the polymerizable mesogenic material.

As a substrate for example a glass or quartz sheet or a plastic film orsheet can be used. It is also possible to put a second substrate on topof the coated mixture prior to and/or during and/or afterpolymerization. The substrates can be removed after polymerization ornot. When using two substrates in case of curing by actinic radiation,at least one substrate has to be, transmissive for the actinic radiationused for the polymerization. Isotropic or birefringent substrates can beused. In case the substrate is not removed from the polymerized filmafter polymerization, preferably isotropic substrates are used.

Preferably at least one substrate is a plastic substrate such as forexample a film of polyester such as polyethyleneterephthalate (PET), ofpolyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC),especially preferably a PET film or a TAC film. As a birefringentsubstrate for example an uniaxially stretched plastic film can be used.For example PET films are commercially available from ICI Corp. underthe trade name Melinex.

The polymerizable mesogenic material can also be dissolved in a solvent,preferably in an organic solvent. The solution is then coated onto thesubstrate, for example by spin-coating or other known techniques, andthe solvent is evaporated off before polymerization. In most cases it issuitable to heat the mixture in order to facilitate the evaporation ofthe solvent.

For preparing an ORF with twisted structure, it is necessary to achieveplanar alignment in the layer of the chiral polymerizable material, i.e.with the helical axis being oriented substantially perpendicular to theplane of the layer. Planar alignment can be achieved for example byshearing the material, e.g. by means of a doctor blade. It is alsopossible to apply an alignment layer, for example a layer of rubbedpolyimide or sputtered SiO_(x), on top of at least one of thesubstrates.

Planar alignment of the polymerizable mesogenic material can also beachieved by directly rubbing the substrate, i.e. without applying anadditional alignment layer. This is a considerable advantage as itallows a significant reduction of the production costs of the opticalretardation film. In this way a low tilt angle can easily be achieved.

For example rubbing can be achieved by means of a rubbing cloth, such asa velvet cloth, or with a flat bar coated with a rubbing cloth. In apreferred embodiment of the present invention rubbing is achieved bymeans of a at least one rubbing roller, like e.g. a fast spinning rollerthat is brushing across the substrate, or by putting the substratebetween at least two rollers, wherein in each case at least one of therollers is optionally covered with a rubbing cloth. In another preferredembodiment of the present invention rubbing is achieved by wrapping thesubstrate at least partially at a defined angle around a roller that ispreferably coated with a rubbing cloth.

The polymerizable composition according to the the present invention mayalso comprise one or more surfactans to improve planar alignment.Suitable surfactants are described for example in J. Cognard, Mol.Cryst. Liq. Cryst. 78, Supplement 1, 1-77 (1981). Particularly preferredare non-ionic surfactants, such as the commercially availablefluorocarbon surfactants Fluorad 171 (from 3M Co.), or Zonyl FSN (fromDuPont). Preferably the polymerizable mixture comprises 0.01 to 5%, inparticular 0.1 to 3%, very preferably 0.2 to 2% by weight ofsurfactants.

The orientation of the mesogenic material depends, inter alia, on thefilm thickness, the type of substrate material, and the composition ofthe polymerizable mesogenic material. It is therefore possible, bychanging these parameters, to control the structure of the ORF, inparticular specific parameters such as the tilt angle and its degree ofvariation.

Thus, for the preparation of the O plate, it is possible to adjust thealignment profile in the direction perpendicular to the film plane byappropriate selection of the ratio of monoreactive mesogenic compounds,i.e. compounds with one polymerizable group, and direactive mesogeniccompounds, i.e. compounds with two polymerizable groups.

For an O plate with strong splay, i.e. a large variation of the tiltangle throughout the thickness of the film, preferably the ratio ofmono- to direactive mesogenic compounds should be in the range of 6:1 to1:2, preferably 3:1 to 1:1, especially preferably about 3:2.

Another effective means to adjust the desired splay geometry is to use adefined amount of dielectrically polar polymerizable mesogenic compoundsin the polymerzable mesogenic material. These polar compounds can beeither monoreactive or direactive. They can be either dielectricallypositive or negative. Most preferred are dielectrically positive andmonoreactive mesogenic compounds.

The amount of the polar compounds in the mixture of polymerizablemesogenic material is preferably 1 to 80%, especially 3 to 60%, inparticular 5 to 40% by weight of the total mixture.

Polar mesogenic compound in this connection means a compound with one ormore polar groups as defined above. Especially preferred aremonoreactive polar compounds selected from formulae Ia to Ic givenabove.

Furthermore, these polar compounds preferably exhibit a high absolutevalue of the dielectric anisotropy Δε, which is typically higher than1.5. Thus, dielectrically positive compounds preferably exhibit Δε>1.5and dielectrically negative polar compounds preferably exhibit Δε<−1.5.Very preferred are dielectrically positive polar compounds with Δε>3, inparticular with Δε>5.

Polymerization of the polymerizable mesogenic material takes place byexposing it to heat or actinic radiation. Actinic radiation meansirradiation with light, like UV light, IR light or visible light,irradiation with X-rays or gamma rays or irradiation with high energyparticles, such as ions or electrons. Preferably polymerization iscarried out by UV irradiation.

As a source for actinic radiation for example a single UV lamp or a setof UV lamps can be used. When using a high lamp power the curing timecan be reduced. Another possible source for actinic radiation is alaser, like e.g. a UV laser, an IR laser or a visible laser.

The polymerization is carried out in the presence of an initiatorabsorbing at the wavelength of the actinic radiation. For example, whenpolymerizing by means of UV light, a photoinitiator can be used thatdecomposes under UV irradiation to produce free radicals or ions thatstart the polymerization reaction.

When curing polymerizable mesogens with acrylate or methacrylate groups,preferably a radical photoinitiator is used, when curing polymerizablemesogens vinyl and epoxide groups, preferably a cationic photoinitiatoris used.

It is also possible to use a polymerization initiator that decomposeswhen heated to produce free radicals or ions that start thepolymerization.

As a photoinitiator for radical polymerization for example thecommercially available Irgacure 651, Irgacure 184, Darocure 1173 orDarocure 4205 (all from Ciba Geigy AG) can be used, whereas in case ofcationic photopolymerization the commercially available UVI 6974 (UnionCarbide) can be used.

The polymerizable mesogenic material preferably comprises 0.01 to 10%,very preferably 0.05 to 5%, in particular 0.1 to 3% of a polymerizationinitiator. UV photoinitiators are preferred, in particular radicalic UVphotoinitiators.

The curing time is depending, inter alia, on the reactivity of thepolymerizable mesogenic material, the thickness of the coated layer, thetype of polymerization initiator and the power of the UV lamp. Thecuring time according to the invention is preferably not longer than 10minutes, particularly preferably not longer than 5 minutes and veryparticularly preferably shorter than 2 minutes. For mass productionshort curing times of 3 minutes or less, very preferably of 1 minute orless, in particular of 30 seconds or less, are preferred.

In addition to polymerization initiators the polymerizable material mayalso comprise one or more other suitable components such as, forexample, catalysts, stabilizers, chain-transfer agents, co-reactingmonomers or surface-active compounds. In particular the addition ofstabilizers is preferred in order to prevent undesired spontaneouspolymerization of the polymerizable material for example during storage.

As stabilizers in principal all compounds can be used that are known tothe skilled in the art for this purpose. These compounds arecommercially available in a broad variety. Typical examples forstabilizers are 4-ethoxyphenol or butylated hydroxytoluene (BHT).

Other additives, like e.g. chain transfer agents, can also be added tothe polymerizable material in order to modify the physical properties ofthe inventive polymer film. When adding a chain transfer agent, such asmonofunctional thiol compounds like e.g. dodecane thiol ormultifunctional thiol compounds like e.g. trimethylpropanetri(3-mercaptopropionate), to the polymerizable material, the length ofthe free polymer chains and/or the length of the polymer chains betweentwo crosslinks in the inventive polymer film can be controlled. When theamount of the chain transfer agent is increased, the polymer chainlength in the obtained polymer film is decreasing.

It is also possible, in order to increase crosslinking of the polymers,to add up to 20% of a non mesogenic compound with two or morepolymerizable functional groups to the polymerizable materialalternatively or in addition to the di- or multifunctional polymerizablemesogenic compounds to increase crosslinking of the polymer.

Typical examples for difunctional non mesogenic monomers arealkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to 20 Catoms. Typical examples for non mesogenic monomers with more than twopolymerizable groups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

In another preferred embodiment the mixture of polymerizable materialcomprises up to 70%, preferably 3 to 50% of a non mesogenic compoundwith one polymerizable functional group. Typical examples formonofunctional non mesogenic monomers are alkylacrylates oralkylmethacrylates.

It is also possible to add, for example, a quantity of up to 20% byweight of a non polymerizable liquid-crystalline compound to adapt theoptical properties of the optical retardation film.

In some cases it is of advantage to apply a second substrate to aidalignment and exclude oxygen that may inhibit the polymerization.Alternatively the curing can be carried out under an atmosphere of inertgas. However, curing in air is also possible using suitablephotoinitiators and high UV lamp power. When using a cationicphotoinitiator oxygen exclusion most often is not needed, but watershould be excluded. In a preferred embodiment of the invention thepolymerization of the polymerizable mesogenic material is carried outunder an atmosphere of inert gas, preferably under a nitrogenatmosphere.

To obtain a polymer film with the desired molecular orientation thepolymerization has to be carried out in the liquid crystal phase of thepolymerizable mesogenic material. Therefore, preferably polymerizablemesogenic compounds or mixtures with low melting points and broad liquidcrystal phase ranges are used. The use of such materials allows toreduce the polymerization temperature, which makes the polymerizationprocess easier and is a considerable advantage especially for massproduction.

The selection of suitable polymerization temperatures depends mainly onthe clearing point of the polymerizable material and inter alia on thesoftening point of the substrate. Preferably the polymerizationtemperature is at least 30 degrees below the clearing temperature of thepolymerizable mesogenic mixture.

Polymerization temperatures below 120° C. are preferred. Especiallypreferred are temperatures below 90° C., in particular temperatures of60° C. or less.

The invention is further explained by the following examples. Therein,the following abbreviations are used:

θ tilt angle [degrees] φ twist angle [degrees] p helical pitch [nm]n_(e) extraordinary refractive index (at 20° C. and 589 nm) n_(o)ordinary refractive index (at 20° C. and 589 nm) ε_(∥) dielectricconstant parallel to the long molecular axis (at 20° C. and 1 kHz) E_(⊥)dielectric constant perpendicular to the long molecular axis (at 20° C.and 1 kHz) K₁₁ first elastic constant K₂₂ second elastic constant K₃₃third elastic constant V_(on) threshold voltage [V] V_(off) saturationvoltage [V] d layer thickness [μm]

COMPARISON EXAMPLE A

An uncompensated standard type TN-LCD device of the E mode as depictedin FIG. 1 a, comprising a TN cell 1 and a pair of linear polarizers 2,2′has the following parameters

n_(e)  1.5700 n_(o)  1.4755 ε_(⊥)  3.5 ε_(∥) 10.8 K₁₁ 11.7 K₂₂  5.7 K₃₃15.7 d  5.25 μm pre-tilt  2 V_(on)  4.07 V V_(off)  1.56 V

FIG. 4 a depicts the isocontrast plot of the display, showing ranges ofidentical contrast in steps of 10%. The isocontrast plots are measuredas luminance at V_(on)/luminance at V_(off).

FIGS. 4 b and 4 c show 8 grey levels (given as transmission versusviewing angle), on a linear luminance scale in horizontal and verticalviewing planes, respectively. Ideally, the grey level lines should beparallel, where they cross, grey level inversion occurs. The latter is aserious disadvantage especially for the darker grey levels. It can beseen in FIG. 4 b that levels 7 and 8 are very poor even at low anglessuch as 30° in horizontal direction, and in FIG. 4 c that the levelscross at angles of 30° and higher in vertical direction.

The polarisers can be any standard polariser used in normal LCDdisplays.

V_(on) V_(off) correspond to values generally adopted in TN and STN-LCDdisplays.

COMPARISON EXAMPLE B

An uncompensated standard type TN-LCD device of the O mode as depictedin FIG. 1 b, comprising a TN cell 1 and a pair of linear polarizers2,2′, has the parameters as given in comparison example A.

FIG. 5 a depicts the isocontrast plot of the display, showing ranges ofidentical contrast in steps of 10%. The isocontrast plots are measuredas luminance at V_(on)/luminance at V_(off).

FIGS. 5 b and 5 c show the grey levels in horizontal and verticalviewing planes, respectively. It can be seen in FIG. 5 b that levels 7and 8 are very poor even at low angles such as 30° in horizontaldirection, and in FIG. 5 c that the levels cross at angles of 30° andhigher in vertical direction.

EXAMPLE 1

A compensated TN-LCD device of the O mode according to the presentinvention as depicted in FIG. 2 a consists of a TN cell 1 with a liquidcrystal layer in a twisted nematic state, a pair of linear polarizers2,2′, two splayed O plates 3,3′, two planar A plates 4,4′, and fournegative C plates 5,5′ serving as substrates for the O plates and Aplates. The stack format of the optical components corresponds to type[H] of table 1 above.

The TN cell 1 and the polarizers 2,2′ are as defined in comparisonexample A.

Thee O plates 3,3′ exhibit a splayed structure with the tilt angle θranging from θ_(min) on one surface to θ_(max) on the opposite surface.

The parameters of the O plates 3,3′ are as follows

θ_(min)  2° θ_(max) 88° θ_(ave) 45° n_(e)  1.610 n_(o)  1.495 d  1.2 μmretardation 70 nm

The parameters of the planar A plates 4,4′ are as follows

n_(e)  1.610 n_(o)  1.495 d′  0.91 μm retardation 105 nm

In the display device according to example 1, the orientation directionsof the optical axes of the individual optical films within the filmplane are given in table 2 below. For a better understanding, theorientation directions of 0°, 90°, 180° and 270° are also depicted bythe arrows on the left side of FIGS. 2 a, 2 b and 3.

Table 2—Orientation Direction of the Optical Axes of IndividualComponents in a Display According to Example 1

left polarizer 2  45° O plate 3 225° planar A plate 4 135° planar Aplate 4′ 225° O plate 3′ 135° right polarizer 2′ 315°

FIG. 6 a shows the isocontrast plot of the display, FIGS. 6 b and 6 cshow the grey levels (transmission vesus viewing angle) in horizontaland vertical directions respectively.

In the isocontrast plot FIG. 6 a it can be seen that the display has aviewing angle that is significantly larger in horizontal direction,compared to the uncompensated display of example B, and is also slightlyimproved in vertical direction. In FIG. 6 b and 6 c it can be seen thatthe grey levels 7 and 8 in horizontal direction are significantlyimproved compared to the uncompensated display of example B, and arealso improved in vertical direction at negative angles.

EXAMPLE 2

A compensated TN-LCD device of the E mode according to the presentinvention as depicted in FIG. 2 b consists of a TN cell 1 with a liquidcrystal layer in a twisted nematic state, a pair of linear polarizers2,2′, two splayed O plates 3,3′, two planar A plates 4,4′, and fournegative C plates 5,5′ serving as substrates for the O plates and Aplates. The stack format of the optical components corresponds to type[H] of table 1 above.

The parameters and orientations of the individual components are asgiven in example 1.

FIG. 7 a shows the isocontrast plot of the display. FIGS. 7 b and 7 cshow the grey levels in horizontal and vertical directions respectively.

In the isocontrast plot FIG. 7 a it can be seen that the display has aviewing angle that is larger in both horizontal and vertical direction,compared to the uncompensated display of example A. In FIGS. 7 b and 7 cit can be seen that the grey levels 7 and 8 in horizontal direction areimproved compared to the uncompensated display of example A, and areimproved in vertical direction at negative angles.

EXAMPLE 3

A compensated TN-LCD device of the E mode according to the presentinvention comprises the individual components and the stack format asshown in FIG. 2 b.

The display according to example 3 relates to a preferred embodiment ofthe present invention, wherein the O plates 3,3′ and the planar A plates4,4′ exhibit the same retardation.

The thickness of the O plates is 1.427 μm.

The thickness of the planar A plates is 0.711 μm.

The retardation of the C plates and planar A plates is 82 nm.

The other parameters are as given in example 1.

The orientations of the individual components are as given in table 2.

FIG. 8 a shows the isocontrast plot of the display, FIGS. 8 b and 8 cshow the grey levels in horizontal and vertical directions respectively.It can be seen that, compared to the uncompensated display of example A,the viewing angle is significantly enlarged. The 100-1 isocontrast areais +/−50° in the horizontal direction and +/−25° in the verticaldirection. The 10-1 isocontrast area is +/−60° in the horizontaldirection and +/−60° in the vertical direction. The grey levels areimproved both in horizontal and vertical direction.

EXAMPLES 4-10

A series of compensated TN-LCD devices of the E mode according to thepresent invention comprise the individual components and the stackformat as shown in FIG. 2 b and corresponding to type [H] of table 1.

In addition, the devices according to examples 4-10 relate to apreferred embodiment, wherein the optical axes 3 b,4 b of the O plate 3and the planar A plate 4 are twisted clockwise, at an angle +δ, and theoptical axes 3 b′,4 b′ of the O plate 3′ and the planar A plate 4′ aretwisted counterclockwise, at an angle −δ, in a plane parallel to thefilm planes and relative to the optical axes of the other opticalelements in the display.

For example, in the device according to example 4 the orientationdirections of the optical axes of the polarizers 2,2′ are as given intable 2 above, whereas the optical axes 3 b and 4 b of the O plate 3 andA plate 4 are oriented at 219° and 129° respectively, and the opticalaxes 3 b′ and 4 b′ of the O plate 3′ and A plate 4′ are oriented at 231°and 141°, respectively. Thus, the angle δ in example is ±6°.

Furthermore, in the displays according to examples 4-10 the O plates andthe planar A plates exhibit the same retardation.

The different parameters of the individual components in the displaydevices according to examples 4-10 are given in table 3 below. The otherparameters are as given example 1.

In table 3, d_(O) and d_(A) denote the thickness of the O plates and theplanar A plates in μm, respectively, ret. denotes the retardation of theO and planar A plates (in nm), θ_(ave) denotes the average tilt angle inthe splayed O plates (in °), δ denotes the angle (in °) at which theoptical axes of the O and planar A plates are twisted relative to theother optical components as described above, and V_(off) denotes thesaturation voltage of the TN cell (in V).

TABLE 3 Parameters of the display components of example 4-10 Ex. d_(O)d_(A) ret. θ_(ave) δ V_(off) 4 1.427 0.711 82 45 ±6 4.07 5 1.22 0.613 7045 ±9 4.94 6 1.22 0.613 70 49 ±9 4.94 7 1.356 0.67 77 45 ±9 4.5 8 1.3560.67 77 47 ±9 4.5 9 1.55 0.768 89 45 ±6.5 3.5 10  1.55 0.768 89 41 ±6.53.5

FIGS. 9, 10, 11, 12, 13, 14 and 15 show the isocontrast plots of thedisplays according to examples 4, 5, . . . , 10, respectively.

It can be seen that, compared to the uncompensated display of example A,the viewing angle is significantly enlarged, and the grey levels areimproved both in horizontal and vertical direction.

The horizontal and vertical angles for the 100-1 and 10-1 isocontrastareas of the displays according to examples 4-10 are given in table 4.

TABLE 4 Angles (°) of isocontrast areas for displays of example 4-10100-1 isocontrast 10-1 isocontrast Ex. FIG. horizontal verticalhorizontal vertical 4  9a ±50 +60/−35 ±60 +60/−55 5 10a ±40 +60/−25 ±60+60/−40 6 11a ±50 +60/−25 ±60 +60/−40 7 12a ±45 +60/−25 ±60 +60/−45 813a ±50 +60/−25 ±60 +60/−45 9 14a ±40 +60/−35 ±60 ±60 10  15a ±50+50/−30 ±60 ±60

EXAMPLE 11

A compensated TN-LCD device of the O mode according to the presentinvention as depicted in FIG. 3 consists of a TN cell 1 with a liquidcrystal layer in a twisted nematic state, a pair of linear polarizers2,2′, two splayed O plates 3,3′, two planar A plates 4,4′, two negativeC plates 5,5′ serving as substrates for the O plates, and two highlytwisted A plates 6,6′ that have the optical performance of a negative Cplate and are situated between O plate 3 and A plate 4, and between Oplate 3′ and A plate 4′, respectively. The stack format of the opticalcomponents corresponds to type [H] of table 1 above.

In the device according to example 11, the optical axes of the O plates3,3′ and the A plates 4,4′ are twisted relative to the other opticalcomponents at an angle δ of ±6° as defined above. The orientation of theother components is as given in table 2, example 1.

The parameters of the highly twisted A plates 6,6′ are as follows

n_(e)  1.610 n_(o)  1.495 d″  3.0 μm pitch p 200 nm

The average tilt angle θ_(ave) of the O plates is 45°.

The thickness of the O plates is 1.427 μm.

The thickness of the planar A plates is 0.711 μm.

The retardation of the O plates and planar A plates is 82 nm.

V_(off) of the TN cell is 4.07V.

The other parameters are as given in example 1.

FIG. 16 a shows the isocontrast plot of the display, FIGS. 16 b and 16 cshow the grey levels in horizontal and vertical directions respectively.It can be seen that, compared to the uncompensated display of example B,the viewing angle is significantly enlarged. The 100-1 isocontrast areais +/−50° in the horizontal direction and +60°/−40° in the verticaldirection. The 10-1 isocontrast area is +/−60° in the horizontaldirection and +/−60° in the vertical direction. The grey levels areimproved both in horizontal and vertical direction.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various conditions andusages.

1. An optical compensator for liquid crystal displays comprising: atleast one O plate retarder, at least one planar A plate retarder, and atleast one negative C plate retarder, wherein the A plate and the O platehave substantially the same retardation.
 2. The optical compensatoraccording to claim 1, comprising one O plate, one planar A plate and twonegative C plates.
 3. The optical compensator according to claim 1,comprising one O plate, one planar A plate and one negative C plate,with the C plate situated between the O plate and the planar A plate. 4.The optical compensator according to claim 1, wherein the average tiltangle θ_(ave) in said O plate retarder is 2 to 88°.
 5. The opticalcompensator according to claim 1, wherein the tilt angle in said O plateretarder varies monotonously in a direction perpendicular to the planeof the film from a minimum value θ_(min) at one surface of the film to amaximum value θ_(max) at the opposite surface of the film.
 6. Theoptical compensator according to claim 5, wherein θ_(min) is 0 to 80°.7. The optical compensator according to claim 5, wherein θ_(max) is 10to 90°.
 8. The optical compensator according to claim 1, wherein thethickness of said O plate and/or planar A plate is 0.1 to 10 μm.
 9. Theoptical compensator according to claim 1, wherein the opticalretardation of said O plate is 20 to 300 nm.
 10. The optical compensatoraccording to claim 1, wherein the optical retardation of said planar Aplate is 20 to 300 nm.
 11. The optical compensator according to claim 1,wherein the O plate comprises a linear or crosslinked polymerized liquidcrystalline material with a tilted or splayed structure.
 12. The opticalcompensator according to claim 1, wherein the planar A plate comprises alinear or crosslinked polymerized liquid crystalline material with aplanar structure.
 13. The optical compensator according to claim 1,wherein at least one of the C plates is a negatively birefringentpolymer film.
 14. The optical compensator according to claim 13, whereinsaid polymer film is a negatively birefringent TAC or DAC film.
 15. Theoptical compensator according to claim 1, wherein the C plate comprisesa linear or crosslinked polymerized chiral liquid crystalline materialwith a helically twisted structure.
 16. The optical compensatoraccording to claim 15, wherein the helical pitch of the chiral liquidcrystalline material is said C plate is less than 250 nm.
 17. A liquidcrystal display device comprising the following elements a liquidcrystal cell formed by two transparent substrates having surfaces whichoppose each other, an electrode layer provided on the inside of at leastone of said two transparent substrates and optionally superposed with analignment layer, and a liquid crystal medium which is present betweenthe two transparent substrates, a polarizer arranged outside saidtransparent substrates, or a pair of polarizers sandwiching saidsubstrates, and at least one optical compensator according to claim 1being situated between the liquid crystal cell and at least one of saidpolarizers, it being possible for the above elements to be separated,stacked, mounted on top of each other, coated on top of each other orconnected by means of adhesive layers.
 18. A liquid crystal displaydevice according to claim 17, which is a TN, HTN or STN display.
 19. Anoptical compensator for liquid crystal displays comprising: at least oneO plate retarder, at least one planar A plate retarder, and at least onenegative C plate retarder, wherein the A plate and the O plate have thesame retardation.
 20. An optical compensator for liquid crystal displayscomprising: at least one O plate retarder, at least one planar A plateretarder, and exactly two negative C plate retarders.
 21. An opticalcompensator for liquid crystal displays comprising at least O plateretarder, and at least one planar A plate retarder, and at least onenegative C plate retarder, wherein the C plate is situated between the Oplate and the planar A plate.